This invention relates generally to methods and apparatus for evaluating subsurface formations and more specifically relates to methods and apparatus for determining characteristics of clay-bearing geological formations by raidoactivity well logging.
In the oil and gas exploration industry, subsurface formations are investigated to determine if hydrocarbon resources may be found therein. When a subsurface reservoir containing hydrocarbon-bearing formations is discovered, it is important to analyze the reservoir formations and to evaluate the practical and economic feasibility of producing the hydrocarbon resources therefrom. Many factors may be considered in analyzing the producibility of the formations within a reservoir; of particular interest are characteristics such as the reservoir porosity, representing the pore volume of the reservoir formations, the fluid saturation characteristics of the reservoir, representing the fraction of the formation pore volume filled with specific types of fluids, and the reservoir permeability, or the ease with which the fluids flow through the formations. Such reservoir characteristics may be determined through analysis of data relating to various subsurface parameters, such data obtained through use of well logs and/or "core" samples recovered from the subsurface formations. Because reservoir characteristics may change throughout the vertical extent of a reservoir due to the geological composition of the formations contained therein, well log data offers an advantage over core data in that well log data may typically be obtained generally continuously throughout the vertical extent of the reservoir in a manner which is time consuming and difficult to duplicate through reliance upon core samples.
Certain reservoirs, specifically those reservoirs containing shale-bearing formations, may be difficult to fully evaluate using conventional well log analysis techniques. In shaly reservoirs, accurate log evaluation depends on a correct determination of water saturation and clay characteristics. Key parameters include clay volume, cation exchange capacity and the sensitivity of the clays to reservoir stimulation and production. Shales typically contain a predominance of clay minerals of differing types intermixed with a variety of non-clay minerals such as quartz, feldspar, mica, and carbonates. Some clay materials found in subsurface formations are illite, kaolinite, chlorite and montmorillonite. Because these non-clay minerals are essentially inseparably intermixed with the clay minerals and because it is the type of clay minerals within the shales which, directly or indirectly, most significantly affect reservoir properties, the term "clay" is used herein essentially synonymously with "shale", thus including these non-clay minerals, and the "clays" are described in terms of the different clay minerals contained therein, the producibility characteristics of clay-bearing formations are significantly affected by the volumes of clay contained therein. Further, different types of clays and their modes of distribution within the reservoir formations affect reservoir properties differently, even with a consistent volume of clay present. Additionally, the type and volume of clay and the type of distribution thereof may change continually throughout the reservoir formations, thereby causing significant variations in various parameters related thereto. For example, the volume of "bound water" within a formation, that volume of formation water which is electrochemically bound to the clay minerals within the formation, is functionally related to both the porosity and the permeability of the formation and varies significantly in response to the type and volume of clay and the mode of distribution thereof within the formation.
Clay corrections typically assume that the clay deposited during the various phases of a continuous sedimentation cycle has the same composition through the complete cycle. By assuming that the clay materials are all equivalent, clay corrections can more easily be calculated and applied. However, results have been found to be unrealistic under some formation conditions resulting in an appraisal which has been too pessimistic in some zones and which may condemn some zones of commercial significance. Such corrections do not take into account the fact that influence of clay volume of formations measurements is nonlinear due to different cation exchange capacities of various clay minerals. Cation exchange is the reaction whereby hydrated, positively charged ions of a solid, such as clay, are exchanged, equivalent for equivalent, for cations of like charge in solution.
In the evaluation of the producibility of a reservoir it is not uncommon to rely upon models based upon various geological parameters, including bound water and porosity data, to describe certain reservoir characteristics, such as fluid saturation and permeability. The accuracy of the characteristics described by the models is clearly dependent upon the accuracy of the data contained therein. Therefore, because of the above-described effects of clay type, volume, and distribution upon reservoir parameters, it is important to continuously evaluate such clay parameters throughout the reservoir.
Another means known to the prior art of estimating various reservoir parameters has been to utilize a correlation, such as a graphic crossplot, of two well logs, such as a bulk density log and an acoustic travel time log, or a bulk density log and a neutron log, to establish a graph upon which is indicated a line representing reservoir formations essentially free of shale or clay, or a 0% clay volume, and a single point, determined from the two logs, representing a 100% shale or clay volume within the reservoir, the coordinates of such point being the response values of each log in such 100% clay environment. Other responses of the logs are then scaled between these two limits to estimate the clay volume at other horizons within the reservoir. Because of the differing responses of the logging instruments to differing clay types and distributions within the reservoir formations, such a method may lead to significant errors in the volume of clay determined at any given depth horizon within the formations. Further, the coordinates or log response values representing 100% clay are also taken as constants, at least within a given portion of the reservoir, and additional reservoir parameters are determined in response thereto. By establishing these log response values as constants, such a method again fails to account for the aforementioned clay types and distributions within the formations and the differing responses of the logging instruments thereto, and is therefore prone to yield further erroneous data regarding those formations.
One means known in the prior art of estimating formation parameters is described in U.S. Pat. No. 4,263,509. This method utilizes a high-resolution, gamma ray spectrometer incorporated in a well logging instrument to derive measurements of characteristic formation radiations which can be correlated to formation data derived from core samples taken from the well. The functional relation between the radiation measurements and the core data can be utilized in evaluating radioactivity data derived from other formations in the same geologic region. Due to the reliance upon core data as used by this technique it has proven difficult and time consuming to derive a functional relationship for use through the vertical extent of a well.
Accordingly, the present invention provides methods and apparatus for in situ determination of formation characteristics without the use of cores or additional formation measurements. Such formation characteristics include a generally continuous volume of clay determination, the cation exchange capacity of the formations and an indicator of the sensitivity of the reservoir to damage from stimulation and production.